1. Field of the Invention
The present invention relates to a method and a system for reconnecting an inverter to rotating motors, and more particularly to a method and a system for connecting an inverter, which is now controlling induction motors at predetermined frequency and voltage, to the motors rotating due to the inertia force of the motor rotor, after the inverter has once been disconnected from the induction motors because of power failure or motor accident. In the method according to the present invention, the inverter is connected to rotating motors again in accordance with forward control method or open-loop control method, in place of feedback control method or closed-loop control method.
2. Description of the Prior Art
Induction motors are usually driven by an inverter. The inverter converts a DC power source rectified by a bridge-connected semiconductor rectifier and a capacitor into an AC power source suitable for driving the induction motors at predetermined frequency and torque. The rotational speed of an induction motor can effectively be controlled by the inverter in accordance with pulse width modulation method (described later).
In order to continuously drive the induction motor, a power supply equipment to protect against interruption of service is usually incorporated in the induction motor driving system including the inverter. In such a system as described above, in case commercial AC power source fails, power interruption is first detected by a power monitor relay and then the inverter is immediately disconnected from the induction motor in order to protect the inverter elements from being damaged by a surge of current applied from the motor to the inverter. Thereafter (after one or two seconds), the power supply equipment which protects against interruption is again connected to the motor driving system. Under these conditions, since the induction motor is still rotating due to the inertia force of the motor rotor, it is indispensable to synchronously reconnect the inverter to the induction motor. In other words, the inverter must be reconnected to the rotating motor while matching inverter frequency to motor speed or inverter phase to motor back electromotive force phase. Otherwise, a current surge will be generated by the motor and will damage the inverter elements, thus resulting in failure of reconnection between the inverter and the induction motor.
Further, there exists the situation where a plurality of parallel-connected induction motors are driven by a single inverter. In such a system as described above, in case one of the motors is connected to ground by accident (ground fault), a current surge discharged from the smoothing capacitor in the rectifier is sensed by a current transformer in order to immediately disconnect the inverter elements from the induction motors; that is, a circuit breaker arranged in the main circuit of induction motors is immediately opened. In this state, since power regenerated by the kinetic energy of the remaining normal induction motors is applied to the abnormal ground fault induction motor, a fuse arranged between the inverter and the abnormal motor is blown out to isolate the abnormal motor from the motor driving system. In response to the fuse melt signal, the inverter is restarted. Under these conditions, similarly, since the induction motor is still rotating due to the inertia force of the motor rotor, it is indispensable to close the circuit breaker arranged in the main circuit of the induction motors when the inverter is synchronized with the induction motors, that is, when inverter phase matches motor back electromotive force phase.
In the prior art system for reconnecting the inverter to rotating induction motors, a tachometer is convertionally attached on the induction motor side in order to detect the rotational speed of the induction motor. In more detail, motor rotational speed or motor back electromotive force phase is monitored by the tachometer; and when the monitored motor phase matches inverter phase, the inverter is reconnected to the induction motor. In other words, the reconnecting operation between the inverter and the induction motor is achieved in accordance with the feedback control method or closed-loop control method.
In the prior art system for reconnecting an inverter to a rotating induction motor, however, there exist the following drawbacks: (1) In the case of feedback control method depending upon a tachometer, control response speed is not high. In other words, it takes a relatively-long time to completely synchronize inverter phase with induction motor back electromotive force phase. (2) In use of a tachometer, an additional complicated circuit is necessary to distinguish the rotational directions (clockwise or counterclockwise) of the induction motor. (3) Since the tachometer must be connected to the induction motor independently from the system for reconnecting an inverter to an induction motor, this is rather troublesome to the users.
In this connection, there exists another system for reconnecting an inverter to a rotating synchronous motor, in place of an induction motor. In this case, the synchronization between the inverter and the synchronous motor can be achieved in accordance with the feedback control method by detecting back electromotive force generated from the synchronous motor rotating due to an inertia force of the motor rotor. However, since the time interval during which the back electromotive force is being generated is relatively short in the case of a synchronous motor and further since it is necessary to perfectly match the back electromotive force phase of the synchronous motor to inverter phase, it is rather difficult to stably reconnect the inverter to the rotating synchronous motor.